Waveguides are commonly used in a number of applications and are particularly suited for transmission of signals in the microwave frequency range. This transmission may be between an antenna, often mounted on a tall tower, and base station equipment located in a shelter at ground level, for example. In general, a waveguide consists of a hollow metallic tube of defined cross-section. Commonly used cross-sectional shapes include rectangular, circular and elliptical.
Each waveguide has a minimum frequency for transmission of signals (the “cut off frequency”). This frequency is primarily a function of the dimensions and cross-sectional shape of the particular waveguide, and is different for different wave modes.
In a dominant mode waveguide, the frequency range of operation of the waveguide is selected such that only the fundamental wave mode (the “dominant mode”) can be transmitted by the waveguide. For example, in a rectangular dominant mode waveguide, the frequency range of operation is typically between 1.25 and 1.9 times the cut off frequency of the dominant mode (the H10 mode). In a typical rectangular waveguide, where the aspect ratio is generally about 0.5, higher order wave modes (e.g. the H01 and H20 modes) are transmitted only above two times the cut off frequency of the dominant wave mode. Thus, this restriction of the frequency range of operation prevents propagation of any wave mode other than the dominant wave mode.
In an overmoded waveguide, the signal frequency is significantly higher than the cut off frequency. For example, in some overmoded elliptical waveguides, the signal is transmitted in the HC11 mode, with a frequency range between 2.43 and 2.95 times the cut off frequency for that mode. In general, this means that an overmoded waveguide has a cross-sectional area that is significantly larger than that of a dominant mode waveguide operating in the same frequency range. The principal reason for using overmoded waveguides is that, as the frequency of the signals increases above the fundamental mode cut off frequency, attenuation of the signals decreases. This decreased attenuation makes use of overmoded waveguides beneficial in some applications despite the problems with these waveguides, described below.
The difference between the signal frequency and the cut off frequency in an overmoded waveguide also means that one or more higher modes are able to propagate in the waveguide, since the operating frequency range is greater than the cutoff frequencies of those modes. It is a significant challenge to operate an overmoded waveguide without disturbing the signal (i.e. the fundamental mode). Any disturbance of this signal may result in the conversion of fundamental mode signals to unwanted higher modes, these unwanted modes propagating in the waveguide and converting back to fundamental mode signals. As the different modes travel at different velocities within the waveguide, such conversion and reconversion back and forth between the modes is a problematic source of noise and signal distortion.
Therefore, it is desirable to minimize mode conversion within the overmoded waveguide, and in particular at any discontinuities in the waveguide structure. Design of transitions for transitioning between an overmoded waveguide and another waveguide is therefore particularly important.
Waveguides are typically coupled at some point. Generally, standard interfaces are dominant moded, so that any system using overmoded waveguide will generally need a first transition from a first (dominant mode) standard interface to overmoded waveguide, and a second transition from overmoded waveguide to a second (dominant mode) standard interface. The coupling systems are critical to successful operation of the waveguide system and a number of different transitions, with a number of different internal shapes, have been used for transitioning between waveguides.
One prior transition for connecting a rectangular dominant mode waveguide to an elliptical overmoded waveguide consists of a straight elliptical cylinder intersecting a tapered rectangular pyramid. The elliptical cylinder has dimensions roughly matching those of the overmoded waveguide, while the rectangular pyramid matches the dimensions of the dominant mode waveguide at one end and broadens linearly until it intersects the ellipse. The straight tapers and abrupt changes in angle cause significant generation of unwanted higher modes.
This transition also uses a mode filter supported by slots running along the transition's internal walls. The mode filter uses a resistive element such as carbon or another resistive pigment that has been printed on a dielectric substrate. The resistivity of the coating is around 1000 Ohms/square.
In general, it is difficult to transition between a dominant mode waveguide and an overmoded waveguide because of the large difference in dimensions of the two waveguides and the need to avoid excessive mode conversion. The transition is one of the largest sources of mode conversion and therefore of signal distortion in the waveguide system.
Waveguide components such as waveguide transitions, joints, bends and the like may be formed by electroforming. This process involves electro-deposition of metal through an electrolytic solution onto a metallic surface (the mandrel). A sufficient amount of material is deposited to form a self supporting structure with a surface which matches the mandrel surface very accurately. Modern numerical control technology allows accurate fabrication of mandrels, so that very precisely engineered components can be made.
However, manufacture by this process has been expensive and requires several additional fabrication steps, including trimming and machining steps such as formation of apertures for coupling, o-ring grooves and means to support a mode filter. Therefore, components produced by this method are expensive. The material generally used is copper-based, adding further to the cost. This material is also relatively heavy. Components have also been fabricated in two or more parts. However, this requires expensive assembly procedures and also creates a discontinuity on the internal surface of the waveguide assembly where the two pieces are joined.
It would therefore be desirable to produce a waveguide transition for use with an overmoded waveguide, which results in low mode conversion.
It would also be desirable to produce a waveguide transition for use with an overmoded waveguide which provides effective filtering of higher modes.
It would also be desirable to provide a simple and cost effective method of forming a waveguide component.